Soon after Rochow invented polyorganosiloxanes or “silicones” (U.S. Pat. Nos. 2,258,218-2,258,222), McGregor discovered that heating boric acid together with silicones produced a viscoelastic fluid that became known as “bouncing putty” (U.S. Pat. No. 2,431,878). This remarkable fluid rebounds almost perfectly when dropped on a hard surface yet, like any fluid, it has no fixed shape. More specifically, bouncing putty responds elastically to sudden impacts, but flows slowly in response to prolonged stresses. Bouncing putty has a viscosity that increases with rate of shear, so it is a shear-thickening fluid or, equivalently, a dilatant fluid.
Since its discovery, bouncing putty has been improved and modified in a number of ways. Wright (U.S. Pat. No. 2,541,851) added a filler such as zinc hydroxide to the putty to improve its bounce. Martin (U.S. Pat. No. 2,644,805) showed that bouncing putty can be formed from boric acid and tetramethyl disiloxane diol-1,3. Boot (U.S. Pat. No. 3,177,176) found that adding silica reinforcing filler to the silicones before adding the boron compounds caused the bouncing putty to form more quickly and at a lower temperature during the subsequent heating step. Boot (U.S. Pat. No. 3,213,048) discovered that bouncing putty can be formed at room temperature by adding alkyl borates to silanol-terminated polydimethylsiloxanes (PDMS).
Beers (U.S. Pat. No. 3,350,344) found that adding an ammonium carbonate salt to bouncing putty prevents the putty from flowing under the stress of its own weight and from staining fabrics. Dean (U.S. Pat. No. 3,661,790) prepared glowing bouncing putty by adding activated zinc sulfide and also reduced the putty's density by incorporating small transparent spheres. Kaiser (U.S. Pat. No. 3,677,997) added polyglycols to bouncing putty and thereby reduced its tendency to become tacky upon extended kneading or use. Mastrangelo (U.S. Pat. No. 4,054,714) discloses that adding noble metal particles to bouncing putty renders that putty electrically conducting. Minuto (U.S. Pat. No. 4,371,493) discloses a method for producing bouncing putty from a dimethyl silicone gum, a boron compound, and a reinforcing filler. Christy (U.S. Pat. No. 5,319,021) added discrete elastic particles to bouncing putty to obtain a material that largely recovers its initial form when a deforming stress is removed. Christy (U.S. Pat. No. 5,607,993) subsequently added thermoplastic microspheres to bouncing putty to reduce its average density to approximately 0.6 g/cc.
Bouncing putty is not, however, the only example of boron being added to silicones. Rochow (U.S. Pat. No. 2,371,068) employed boric acid esters as dehydrating agents for silicols. Nicodemus (U.S. Pat. No. 2,442,613) added boric acid or an organic borate to a heat-hardenable silicone to prevent copper from corroding when the silicone is vulcanized onto that copper. McGregor (U.S. Pat. No. 2,459,387) employed boron trifluoride as a dehydrating agent. Upson (U.S. Pat. No. 2,517,945) combined a silanediol with a boronic acid to obtain a thermoplastic copolymer, but noted no unusual viscoelastic properties in the finished copolymer. Dickmann (U.S. Pat. No. 2,721,857) found that adding 0.005 to 0.090 wt % boron compound to unvulcanized silicone elastomer stock improved the handling of that stock and reduced its stickiness, but teaches that “when the boron compound is present in an amount exceeding the upper limit set forth above [0.090 wt %], the physical properties of the resulting silicone elastomer are seriously impaired.”
Nitzsche (U.S. Pat. No. 2,842,521) found that boric acid hydroxyl complexes act as catalysts for the curing of organosiloxane resins, but noted no unusual viscoelastic properties in the finished polymer. Brown (U.S. Pat. No. 2,983,697) added 0.01 to 0.16 wt % boron as tris-triorganosilyl-borates to silicone elastomers to retard crepe hardening, but teaches that “When the amount of boron is greater than 0.16 part per 100 parts of siloxane . . . , the additional boron . . . degrades other physical properties.”
Nitzsche (U.S. Pat. No. 3,050,490) disclosed that adding boron nitride to hydroxyl enblocked polymeric dimethylsiloxane gum, forming the mixture into a tape, and pre-vulcanizing that mixture resulted in a self-adhering tape that could be wound on an object and vulcanized into a homogeneous, unitary tube. Nitzsche (U.S. Pat. No. 3,050,491) disclosed that adding 0.001 to 0.1 wt % boric acid or alkyl borates produced self-adhering material, but teaches that “Larger quantities of boron compound impede the vulcanization and depress the physical properties of the ultimate rubber.” Nitzsche (U.S. Pat. No. 3,070,559) discloses crosslinking agents that can be used to make silicone rubbers and includes without comment in a long list of compounds “esters of boric acid.” Nitzsche (U.S. Pat. No. 3,070,567) then discloses that incorporating 0.1 to 10 wt % of a complex compound of boric acid and a polyhydric alcohol in a silicone base can yield self-adhering tapes that stick to themselves only at elevated temperature.
In a patent on self-adhering silicone rubber, Nitzsche (U.S. Pat. No. 3,230,121) discloses the use of boron-containing self-adhering silicone rubber insulating tape to protect hollow glass articles. He notes that “The silicone rubbers of the present discovery possess the surprising property that the more violent the blow, the greater will be the rebound elasticity. They possess this property in common with the above-mentioned ‘bouncing putty,’ to which they are chemically related.” Nitzsche's comment is made in the context of protecting glassware from impact and is not generalized to any other purpose. Moreover, the silicone rubbers Nitzsche employed in U.S. Pat. No. 3,230,121 are themselves prior art and Nitzsche provides a comprehensive list of prior art patents. The most recent of those prior art patents is Nitzsche's own work: U.S. Pat. No. 3,050,491 (listed in U.S. Pat. No. 3,230,121 as “Ser. No. 9,428, filed Feb. 18, 1960”). In U.S. Pat. No. 3,050,491, Nitzsche teaches against using more than 0.1 wt % boron compounds in silicone elastomers.
Eisinger (U.S. Pat. No. 3,231,542) discloses boron-containing self-adhering silicone rubbers with improved surface characteristics. Fekete (U.S. Pat. No. 3,296,182) incorporates approximately 0.35 wt % boric acid to silicones, along with a titanium compound, to obtain pressure-sensitive adhesive elastomers. Kelly (U.S. Pat. No. 3,330,797) discloses additional boron-containing self-adhering silicone elastomers. Foster (U.S. Pat. No. 3,379,607) added boron compounds to silicones to promote adhesion to surfaces. Proriol (U.S. Pat. No. 3,629,183) discloses boron-containing silicones that vulcanize to form adhesive elastomers on heating. Greenlee (U.S. Pat. No. 3,772,240) found that adding boric acid to silicones improved their adhesion to metals. Wegehaupt (U.S. Pat. No. 3,855,171) incorporates pyrogenically produced mixed oxides of boron and an element selected from the class consisting of silicon, aluminum, titanium and iron in silicones for the purposes of preparing either self-adhering elastomers or bouncing putty.
Maciejewski (U.S. Pat. No. 4,339,339) recognizes that bouncing putty's bounciness makes it unable to absorb energy during sudden impacts. He discloses a boron-containing, non-vulcanizable silicone for use for hydrostatic damping and shock absorption that is able to absorb energy during impacts because it does not exhibit the unusual resiliency of bouncing putty.